Protein kinases are a large family of proteins that play a pivotal role in the regulation of a wide variety of cellular processes, maintaining control over cellular function. Protein tyrosine kinases may be classified as growth factor receptor (e.g. VEGFR, EGFR, PDGFR, FGFR and erbB2) or non-receptor (e.g. c-src and bcr-abl) kinases. Receptor tyrosine kinases (RTKs) play a key role in the regulation of cell proliferation, differentiation, metabolism, migration, and survival. Upon ligand binding, they undergo tyrosine phosphorylation at specific residues in the cytoplasmic tail. This leads to the binding of protein substrates and/or the establishment docking sites for adaptor proteins involved in RTK-mediated signal transduction. When unregulated, receptor tyrosine kinases can contribute to the rise of disease states associated with such aberrant kinase activity.
A partial list of such kinases include abl, AATK, ALK, Akt, axl, bmx, bcr-abl, Blk, Brk, Btk, csk, c-kit, c-Met, c-src, c-fins, CDK1, CDK2, CDK3, CDK4, CDK5, CDK6, CDK7, CDK8, CDK9, CDK10, cRaf1, CSF1R, CSK, DDR1, DDR2, EPHA, EPHB, EGFR, ErbB2, ErbB3, ErbB4, Erk, Fak, fes, FER, FGFR1, FGFR2, FGFR3, FGFR4, FGFR5, Fgr, flt-1, Fps, Frk, Fyn, GSG2, GSK, Hck, ILK, INSRR, IRAK4, ITK, IGF-1R, INS-R, Jak, KSR1, KDR, LMTK2, LMTK3, LTK, Lck, Lyn, MATK, MERTK, MLTK, MST1R, MUSK, NPR1, NTRK, MEK, PLK4, PTK, p38, PDGFR, PIK, PKC, PYK2, RET, ROR1, ROR2, RYK, ros, Ron, SGK493, SRC, SRMS, STYK1, SYK, TEC, TEK, TEX14, TNK1, TNK2, TNNI3K, TXK, TYK2, TYRO3, tie, tie2, TRK, Yes, and Zap70.
Aberrant angiogenesis contributes to some pathological disorders and in particular to tumor growth. VEGF-A (vascular endothelial growth factor A) is a key factor promoting neovascularization (angiogenesis) of tumors. VEGF induces endothelial cell proliferation and migration by signaling through two high affinity receptors, the fms-like tyrosine kinase receptor, flt-1 or VEGFR-1, and the kinase insert domain-containing receptor, KDR or VEGFR-2. The binding of VEGF to VEGFRs stimulates receptor dimerization and activation of the RTK domain. The kinase activity autophosphorylates cytoplasmic receptor tyrosine residues, which then serve as binding sites for molecules involved in the propagation of a signaling cascade.
Disruption of VEGFR signaling is a highly attractive therapeutic target in cancer, as angiogenesis is a prerequisite for all solid tumor growth. A number of drugs inhibiting VEGF signaling, including use of neutralizing antibodies receptor antagonists, small molecule antagonists, have been approved by US Food & Drug Administration (FDA) (“Molecular basis for sunitinib efficacy and future clinical development.” Nature Review Drug Discovery, 2007, 6, 734; Angiogenesis: “an organizing principle for drug discovery?” Nature Review Drug Discovery, 2007, 6, 273).
Hepatocyte growth factor (HGF), also known as scatter factor, is a multifunctional growth factor that promotes cell proliferation, scattering, invasion, survival, and angiogenesis. In order to produce cellular effects, HGF must bind to its receptor, c-Met, a receptor tyrosine kinase. c-Met is overexpressed in a significant percentage of various types of human cancers and is often amplified during the transition between primary tumors and metastasis (“Molecular cancer therapy: can our expectation be MET.” Euro. J. Cancer, 2008, 44, 641-651). c-Met is also implicated in atherosclerosis and lung fibrosis.
Binding of HGF to c-Met leads to receptor phosphorylation and activation of Ras/mitogen-activated protein kinase (MAPK) signaling pathway, thereby enhancing malignant behaviors of cancer cells. Moreover, stimulation of the HGF/c-Met pathway itself can lead to the induction of VEGF expression, itself contributing directly to angiogenic activity. Because of the dual role of c-Met as an adjuvant, pro-metastatic gene for some tumor types and as a necessary oncogene for others, c-Met is a versatile candidate for targeted cancer therapeutic intervention. (“From Tpr-Met to Met, tumorigenesis and tubes.” Oncogene. 2007, 26, 1276; “Drug development of MET inhibitors: targeting oncogene addiction and expedience.” Nature Review Drug Discovery, 2008, 7, 504).
Anti-tumor approaches that target VEGF/VEGFR and HGF/c-Met signaling may circumvent the ability of tumor cells to overcome VEGFR or HGFR inhibition alone and may represent improved cancer therapeutics. Here we describe small molecules that are potent inhibitors of protein tyrosine kinase activity, such as that of, for example, the VEGF receptor KDR and the HGF receptor c-Met, among others.